1. Field of the Invention
The present invention relates to a process for producing hydrogenated polymers including a step of hydrogenating the aromatic rings of a copolymer of an aromatic vinyl compound and a (meth)acrylate in the presence of a catalyst which is composed of palladium supported on the carrier mainly constituted by zirconium oxide. The hydrogenated polymers obtained by the above process exhibit a high transparency, a low birefringence, a high heat resistance, a high surface hardness, a low water absorption, a low specific gravity, a high transfer property and an excellent mold releasability. Particularly, because of their excellence in the properties required in optical materials, the hydrogenated polymers are used in wide applications such as optical lenses, light guide plates, light diffuser plates, optical disk substrates, and display front panels.
2. Description of the Prior Art
In recent years, amorphous plastics such as acrylic resins, methacrylic resins, styrene-based resins, polycarbonate resins and cyclic polyolefin resins have been used in various applications, in particular, there is an increasing demand for the amorphous plastics in the fields of optical materials such as optical lenses and optical disk substrates because of their excellent optical properties. These optical materials have been required to have not only a high transparency but also a high performance excellent in the balance in a high heat resistance, a low water absorption, mechanical properties, etc.
However, the conventional optical materials do not necessarily satisfy all of these properties and still have problems to be solved. For example, polystyrenes are mechanically brittle, large in the birefringence and poor in the transparency. Although having a good heat resistance, polycarbonates have a large birefringence and a poor transparency nearly equal to that of the polystyrenes. Polymethyl methacrylate has a high transparency, but has a poor dimensional stability due to a very high water absorption and a low heat resistance. Polyvinyl cyclohexane obtained by hydrogenating the aromatic ring of polystyrene exhibits a high transparency, but has a low mechanical strength, a poor heat stability and a poor adhesion to other materials (Japanese Patent 3094555, JP 2004-149549A and JP 2003-138078A). In order to improve the adhesion property, there has been proposed a method of mixing an aromatic ring-hydrogenated product of polystyrene, a double bond/aromatic ring-hydrogenated product of a conjugated diene-styrene copolymer, or a saturated hydrocarbon resin (Japanese Patent 2725402). However, the above method requires a complicated operation. Also, there is disclosed that a hydrogenated product, which is obtained by copolymerizing a vinyl aromatic compound such as styrene with an unsaturated dibasic acid such as maleic anhydride and then hydrogenating 30% or more of the aromatic rings of the resultant copolymer, is improved in the transparency and birefringence as compared with polystyrene (JP 7-94496B). However, the proposed hydrogenated product is still insufficient in the optical properties as compared with acrylic resins.
In addition, copolymers of methyl methacrylate (MMA) and styrene (MS resins) exhibit a high transparency and are well-balanced in the dimensional stability, rigidity and specific gravity, but have a large birefringence.
Aromatic ring-hydrogenated products of MS resins (MSH resins), in particular, MSH resins having an MMA copolymerization rate of 50% or higher, are considerably improved in the birefringence and exhibit a good balance in the transparency, heat resistance and mechanical properties as compared with MS resins.
The hydrogenation of the aromatic rings of aromatic polymers is known in the art (DE 1131885A). It has been recognized in the art that the degree of hydrogenation of the aromatic rings should be increased for a high transparency and the degree of hydrogenation should reach near 100%, otherwise a highly transparent resin cannot be obtained. This is because when the degree of hydrogenation of the aromatic rings is low, the resultant hydrogenated polymers form a block structure to reduce the total light transmittance.
A solid catalyst composed of a carrier such as activated carbon, alumina, silica and diatomaceous earth and a metal such as Pd, Pt, Rh, Ru, Re and Ni supported on the carrier is preferably used because of easiness of separation from the reactants. Although many examples are known about the hydrogenation of not only aromatic polymers but also other polymers such as conjugated diene polymers, it has been difficult to obtain a high degree of hydrogenation and a high reaction rate because of a low reactivity due to the high molecular weight. It is also known that the catalytic activity is easy to be reduced when repeatedly used in the reaction. The reduction in the catalytic activity results in a lowered degree of hydrogenation, to impair the transparency of the resins. To improve the reaction activity, the kind, fine pore structure and particle size of carrier have been examined. For example, there have been reported a method of producing a hydrogenated polystyrene having a degree of aromatic ring-hydrogenation of about 70% by using a catalyst composed of palladium supported on a silica carrier having a particle size of less than 100 μm (Japanese Patent 3094555), and a method of producing a hydrogenated polystyrene by using a catalyst formed by supporting Pt and Rh on a silica carrier having a large pore size of exceeding 600 Å (JP 11-504959A). There is also disclosed a hydrogenation method in which the degree of aromatic ring-hydrogenation is kept low while hydrogenating the ethylenically unsaturated bond in a high degree of hydrogenation in the presence of a catalyst obtained by supporting a group VIII metal on a porous carrier. In the porous carrier, 95% or more of the total pore volume is occupied by pores with a pore size of 450 Å, and the surface area of the supported metal occupies within 75% of the surface area of the carrier (Japanese Patent 320057).
JP 2002-521509A reports that the aromatic rings of polystyrene are completely hydrogenated without decreasing the molecular weight in the presence of a catalyst formed by supporting a group VIII metal on a carrier such as silica and alumina in which the pores having a pore size of 100 to 1000 Å occupy 70 to 25% of the total pore volume, and JP 2002-521508A reports that the aromatic rings of polystyrene are completely hydrogenated without decreasing the molecular weight in an ether linkage-containing hydrocarbon in the presence of a commercially available catalyst for the hydrogenation of low-molecular weight compounds, which is formed by supporting a subgroup VIII metal on a carrier such as silica and alumina in which the pores having a pore size of 100 to 1000 Å occupy less than 15% of the total pore volume of the carrier. JP 1-213306A discloses that a catalyst formed by supporting a metal on an oxide of a group IVa element such as titanium oxide and zirconium oxide exhibits a high catalytic activity even when reused in the hydrogenation of a conjugated diene-based polymer such as NBR. However, this patent document describes only the hydrogenation of a conjugated diene-based polymer and is completely silent about the hydrogenation of the aromatic rings. JP 2000-95815A discloses that the unsaturated bond (inclusive of aromatic ring) of an aromatic-conjugated diene copolymer is efficiently hydrogenated without the elution of the metal components in the presence of a catalyst which is obtained by adding an alkali metal or an alkaline earth metal to a carrier in which the pores with a pore size of 100 to 100000 nm occupy 50 to 100% of the total pore volume of the carrier, and then, supporting a platinum group metal on the carrier so that 90% or more of the metal is present within the surface portion of the carrier (depth within 1/10 of the pore diameter).
JP 2003-529646A reports that the catalyst activity can be improved by conducting the hydrogenation after removing catalyst poisons from a polymerization product solution by an activated alumina, and JP 2002-249515A reports that the productivity can be increased by improving the linear reaction rate in a fixed bed reaction.
The hydrogenation of aromatic rings is largely affected by the solvent used because it is a polymer reaction. In the known hydrogenation methods, various reaction solvents such as hydrocarbons, alcohols, ethers and esters are used (JP 2001-527095A). However, hydrocarbons and alcohols are poor solvents for the resins. Ethers such as 1,4-dioxane have a low ignition point and the solvent should be replaced with another solvent such as toluene before the volatilizing extrusion at a high temperature. Further, tetrahydrofuran as the ether solvent is unstable because readily undergoes a ring opening reaction. Esters are safe and relatively stable, and allow the hydrogenation to proceed rapidly. However, esters make the obtained reaction solution and solid resin cloudy, to reduce the transparency. To solve this problem, there has been proposed a method in which a highly transparent hydrogenated aromatic polymer is safely, stably and rapidly produced by conducting the hydrogenation in a mixed solvent of an ether and an alcohol. However, the separation of two kinds of solvents makes the process complicated. Japanese Patent 2890748 discloses that a high transparency can be achieved even at a low degree of hydrogenation by using a mixed solvent of an ether solvent with an alcohol or water. However, the proposed method is not applicable to many polymers because it is effective for only limited aromatic polymers.
As described above, the conventional production of aromatic ring-hydrogenated polymers having high optical properties involves various difficulties and problems. In particular, although the hydrogenated polymers obtained by hydrogenating the aromatic rings of a copolymer of an aromatic vinyl compound and a (meth)acrylate exhibit excellent properties such as a high transparency, low birefringence, high heat resistance, high surface hardness, low water absorption and low specific gravity, no method suitable for stably and rapidly producing such polymers repeatedly or for a long period of time has been provided until now.